00573877 PDF BRITISH STANDARD BS EN 61206 1995 IEC 1206 1993 Ultrasonics — Continuous wave Doppler systems — Test procedures The European Standard EN 61206 1995 has the status of a British Standard BS[.]
BRITISH STANDARD Ultrasonics — Continuous-wave Doppler systems — Test procedures The European Standard EN 61206:1995 has the status of a British Standard BS EN 61206:1995 IEC 1206:1993 BS EN 61206:1995 Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee EPL/87, Ultrasonics, upon which the following bodies were represented: British Dental Association British Institute of Radiology British Medical Ultrasound Society British Society for Rheumatology Department of Health Department of Trade and Industry (National Physical Laboratory) Institute of Laryngology and Otology Institute of Physical Sciences in Medicine Institution of Electrical Engineers This British Standard, having been prepared under the direction of the Electrotechnical Sector Board, was published under the authority of the Standards Board and comes into effect on 15 October 1995 Amendments issued since publication © BSI 01-2000 Amd No The following BSI references relate to the work on this standard: Committee reference EPL/87 Special announcement BSI News May 1995 ISBN 580 24576 Date Comments BS EN 61206:1995 Contents Committees responsible National foreword Foreword Text of EN 61206 List of references © BSI 01-2000 Page Inside front cover ii Inside back cover i BS EN 61206:1995 National foreword This British Standard has been prepared by Technical Committee EPL/87 and is the English language version of EN 61206:1995 Ultrasonics, Continuous-wave Doppler systems — Test procedures, published by the European Committee for Electrotechnical Standardization (CENELEC) It is identical with Technical Report IEC 1206:1993, published by the International Electrotechnical Commission (IEC) The United Kingdom voted against this document being harmonized as an EN, as the IEC Technical Report Type was not intended to be regarded as an International Standard, but only as a prospective standard for provisional application, for guidance on how standards in this field should be used to meet an identified need The IEC Technical Report is due for further review three years after publication, with the options of either extension for a further three years or conversion to an International Standard, or withdrawal The EN will correspondingly be automatically reviewed after a period of five years or earlier depending on the outcome of the IEC review Cross-references Publication referred to Corresponding British Standard EN 61102:1993 (IEC 1102:1991) BS EN 61102:1994 Specification for measurement and characterisation of ultrasonic fields using hydrophones in the frequency range 0.5 MHz to 15 MHz A British Standard does not purport to include all the necessary provisions of a contract Users of British Standards are responsible for their correct application Compliance with a British Standard does not of itself confer immunity from legal obligations Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, the EN title page, pages to 22, an inside back cover and a back cover This standard has been updated (see copyright date) and may have had amendments incorporated This will be indicated in the amendment table on the inside front cover ii © BSI 01-2000 EUROPEAN STANDARD EN 61206 NORME EUROPÉENNE February 1995 EUROPÄISCHE NORM ICS 17.140.50; 11.040.50 Descriptors: Ultrasound, Doppler, continuous wave, test procedure English version Ultrasonics Continuous-wave Doppler systems Test procedures (IEC 1206:1993) Ultrasons Ensembles effet Doppler ondes entretenues Méthodes d’essai (CEI 1206:1993) Ultraschall Dauerschall Doppler System Prüfverfahren (IEC 1206:1993) This European Standard was approved by CENELEC on 1994-12-06 CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member This European Standard exists in three official versions (English, French, German) A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions CENELEC members are the national electrotechnical committees of Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung Central Secretariat: rue de Stassart 35, B-1050 Brussels © 1995 Copyright reserved to CENELEC members Ref No EN 61206:1995 E EN 61206:1995 Foreword The text of the International Standard IEC 1206:1995, prepared by IEC TC 87, Ultrasonics, was submitted to the formal vote and was approved by CENELEC as EN 61206 on 1994-12-06 without any modification The following dates were fixed: — latest date by which the EN has to be implemented at national level by publication of an identical national standard or by endorsement — latest date by which the national standards conflicting with the EN have to be withdrawn (dop) 1995-12-15 (dow) 1995-12-15 Annexes designated “normative” are part of the body of the standard Annexes designated “informative” are given for information only In this standard, Annex ZA is normative and Annex A, Annex B and Annex C are informative Annex ZA has been added by CENELEC Contents Foreword Introduction Section General 1.1 Scope 1.2 Normative reference 1.3 Definitions 1.4 Symbols Section Overall tests of complete systems 2.1 General considerations 2.1.1 Types of Doppler ultrasound systems 2.1.2 Worst case conditions 2.2 Initial conditions 2.2.1 Power supply 2.2.2 Test frequency, general conditions 2.2.3 Working distance 2.2.4 Zero-signal noise level 2.3 Doppler frequency response 2.3.1 Frequency response range 2.3.2 Doppler frequency accuracy 2.3.3 Large-signal performance 2.4 Spatial response 2.4.1 Axial response Page 3 3 4 4 5 6 6 7 8 2.4.2 Lateral response 2.5 Operating frequency 2.5.1 Acoustical measurement 2.5.2 Electrical measurement 2.6 Flow direction separation 2.6.1 Channel separation 2.6.2 Simultaneous flow 2.7 Response to Doppler spectrum 2.7.1 Volume-flow circuits 2.7.2 Maximum-frequency followers Section Special doppler test objects 3.1 Doppler test objects 3.1.1 String Doppler test object 3.1.2 Band Doppler test object 3.1.3 Disk Doppler test object 3.1.4 Piston Doppler test object 3.1.5 Small ball test object 3.1.6 Flow Doppler test object 3.1.7 Water tank (or gel block) Annex A (informative) Description of continuous-wave Doppler ultrasound systems Annex B (informative) Rationale Annex C (informative) Bibliography Annex ZA (normative) Other international publications quoted in this standard with the references of the relevant European publications Figure — Schematic diagram of a string Doppler test object Figure — Schematic diagram of band, disc and piston Doppler test objects Figure — Schematic diagram of a flow Doppler test objects with pump return Figure A.1 — Example of single-channel directional Doppler ultrasound system Figure A.2 — Example of directional Doppler receiver and signal processing Table — Worst case quantities, and corresponding subclause numbers Page 9 9 9 10 10 10 10 10 11 12 12 12 12 13 17 20 21 21 14 15 16 18 19 © BSI 01-2000 EN 61206:1995 Introduction Continuous-wave ultrasonic Doppler flowmeters, velocimeters, or foetal heart detectors are widely used in clinical practice This type of medical ultrasonic equipment measures the Doppler-shift frequency which is the change in frequency of an ultrasound scattered wave caused by relative motion between a scatterer and the ultrasonic transducer This frequency is proportional to the observed velocity, which is the component of the velocity of a scatterer that is directed towards or away from the transducer This technical report describes a range of test methods that may be applied to determine various performance parameters for continuous-wave Doppler ultrasound systems They may also be applied to pulsed Doppler systems although additional tests would also be required The test methods are based on the use of a number of specialised devices such as string, band, disk, piston and flow Doppler test objects These test methods may be considered as falling into one of the following three categories The first is routine quality control tests that can be carried out by a clinician or a technologist to ensure that the system is working adequately or has adequate sensitivity The second is more elaborate test methods, conducted less frequently, such as when the system is suspected of not working properly The third represents tests that would be done by a manufacturer on complete systems, as the basis of type specification of performance Section General 1.1 Scope This technical report describes: — test methods for measuring the performance of continuous-wave ultrasonic Doppler flowmeters, velocimeters, or foetal heart detectors; — special Doppler test objects for determining various performance properties of Doppler ultrasound systems This technical report applies to: — tests made on an overall Doppler ultrasound system; a system which is not disassembled or disconnected; — tests made on continuous-wave Doppler ultrasound systems The same tests can be applied to Doppler ultrasound systems which measure position as well as velocity, such as pulsed and frequency-modulated Doppler systems, although additional tests may then be required © BSI 01-2000 Electrical safety and acoustic output are not covered in this technical report 1.2 Normative reference The following standard contains provisions which, through reference in this text, constitute provisions of this technical report At the time of publication, the edition indicated was valid All standards are subject to revision, and parties to agreements based on this technical report are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below Members of IEC and ISO maintain registers of currently valid International Standards IEC 1102:1991, Measurement and characterisation of ultrasonic fields using hydrophones in the frequency range 0,5 MHz to 15 MHz 1.3 Definitions For the purposes of this technical report, the following definitions apply: 1.3.1 direction sensing; directional descriptor of a type of Doppler ultrasound system which indicates whether scatterers are approaching or receding from the ultrasonic transducer 1.3.2 direction resolving; direction separating descriptor of a type of Doppler ultrasound system in which the Doppler output appears at different output terminals, output channels or output devices depending upon the direction of scatterer motion relative to the transducer 1.3.3 doppler frequency; doppler-shift frequency change in frequency of an ultrasound scattered wave caused by relative motion between the scatterer and the transducer It is the difference frequency between the transmitted and the received wave 1.3.4 doppler output; direct output; doppler frequency output voltage at the Doppler frequency or at Doppler frequencies which activates the output device 1.3.5 doppler output connector electrical connector or that part of a Doppler ultrasound system at which the Doppler output is available for connection to external output devices EN 61206:1995 NOTE Not all Doppler ultrasound systems have a physical connector at which the Doppler output is available 1.4 Symbols 1.3.6 doppler spectrum c I set of Doppler frequencies produced by a Doppler ultrasound system 1.3.7 doppler test object artificial structures used in testing Doppler ultrasound systems They produce ultrasonic reflections that are similar to those produced by the structures on which the Doppler ultrasound systems are to be used NOTE Doppler test objects are often referred to as phantoms 1.3.8 doppler ultrasound system; system equipment designed to transmit and receive ultrasound and to generate a Doppler output from the difference in frequency between the transmitted and received waves 1.3.9 non-directional descriptor of a type of Doppler ultrasound system which is not direction sensing 1.3.10 observed velocity component of the velocity of a scatterer that is directed towards or away from the transducers 1.3.11 operating frequency the ultrasonic or electrical frequency of operation of an ultrasonic transducer forming part of a Doppler ultrasound system 1.3.12 output channel part of a Doppler ultrasound system which functionally represents a particular aspect of the Doppler output NOTE A Doppler ultrasound system may have two output channels, each representing a flow in a particular direction 1.3.13 output device any device included in a Doppler ultrasound system or capable of being connected to it that makes the Doppler output accessible to the human senses is the average speed of sound in a medium is the average speed of the fluid in a flow Doppler test object is the angle between the sound beam and the axis of the tube, string, band or disc in flow, string, band or disc Doppler test objects respectively is the ultrasonic wavelength Section Overall tests of complete systems 2.1 General considerations 2.1.1 Types of Doppler ultrasound systems A major factor that affects performance testing of a Doppler ultrasound system (system) is whether it can be described as directional, non-directional, or as direction resolving Directional or direction sensing refers to a type of system which indicates whether scatterers are approaching or receding from the ultrasonic transducer Non-directional systems not indicate direction of scatterer motion Direction resolving, or direction separating systems provide for Doppler output to appear at different output channels depending upon the direction of scatterer motion Annex A gives descriptions and examples of these different types of systems 2.1.2 Worst case conditions A test method may be applied to determine a particular performance parameter of a system Often a number of quantities can have a bearing on overall performance, each one of which requires the application of a distinct test method Some of these quantities need to be maximised and others need to be minimised in order to obtain the best overall performance Considering overall performance, Table gives the worst case conditions for key quantities appropriate to peripheral vascular systems and the corresponding clause number which describes a suitable test method Table may need modification to be appropriate for other uses As an example, if the noise as measured in 2.2.4 is maximised this will lead to worst case overall performance; conversely, minimising noise will lead to maximised performance The situation for spatial response (see clause 2.4), is discussed in the rationale (see Annex B) © BSI 01-2000 EN 61206:1995 Table — Worst case quantities, and corresponding subclause numbers Worst case is the minimum value of: Quantities Worst case is the maximum value of: Subclause Quantities Subclause Working distance 2.2.3 Noise level 2.2.4 High-frequency response 2.3.1 Low-frequency response 2.3.1 Fixed target effect on sensitivity 2.3.3.2 Distortion 2.3.3.1 Channel separation 2.6.1 Simulator flow error 2.6.2 2.2 Initial conditions 2.2.2 Test frequency, general conditions These clauses describe conditions common to all of the tests given in clauses 2.3 to 2.7, as well as a procedure for locating the appropriate Doppler-shift frequency and distance ranges to be used for these measurements Where a particular type of system may be comprised of various combinations of components, it is intended that each combination should be regarded as a separate system for testing purposes For example, a system may have various transducer options In this case, each transducer and output recording or presentation device connected to the basic electronics will define a different system For tests to be meaningful, all instrument controls, particularly the volume or gain controls, should be recorded during the test An initial nominal test Doppler frequency as specified by the manufacturer, or 1,0 kHz if none is specified, should be obtained by operating the system and transducer with one of the Doppler test objects specified in clause 3.1 The sound beam is directed at the appropriate moving portion of the Doppler test object, whose speed of operation should be adjusted to produce the nominal test Doppler frequency in the Doppler frequency output of the system The transducer should be affixed in a clamp capable of translating the transducer along, and at right angles to, the axis of maximum sensitivity of the system under test Alternatively, the Doppler test object can be moved to cause the same relative displacements In both cases, the mounting should allow the angle of the sound beam emitted by the transducer to be changed relative to the moving portion of the Doppler test object, while allowing the separation of the transducer and the Doppler test object to be changed The separation adjustment should be independent of the angular adjustments so that the true axial response along the sound beam can be measured Where appropriate, and unless otherwise stated, the Doppler-shift frequency and the Doppler output should be observed and measured on each of the outputs provided for the system being tested, with each of the transducers with which it is expected to work It is recommended that the readings be taken at the Doppler output connector if one is available The single-channel output systems usually can be tested by observing their output indication relative to any calibration scales or marks on the system 2.2.1 Power supply To ensure that the stated specifications hold over the range of power supply voltage, tests should be undertaken for the different power line voltages and the worst case test result values reported The power line voltages are to be used at their nominal values and at 10 % above and below the nominal voltage For power line operated systems the worst case values are those obtained after a specified warm-up time Portable battery-operated systems weighing less than one kilogram should be tested with no warm-up and only over the time span sufficient to perform each test to simulate typical use Heavier battery-powered systems should be tested under the same conditions as the power line operated systems For all battery-operated systems the results should be the worst case found over the span of battery voltages from the fully charged condition to a nominal end-of-life voltage Any system tuning or adjustment should be done as specified in the instruction supplied to the user It should be stated whether the nominal life-span of the battery occurs under continuous or intermittent conditions of use This allows the manufacturer to select the intended normal battery life for either intermittent or continuous use © BSI 01-2000 EN 61206:1995 In the tests that use Doppler test objects, as illustrated above, the use of a tissue-equivalent absorber is recommended and described in this technical report This is done to be sure that the signal levels in the system are close to those that will be encountered in practice It is possible to make these tests in a water bath without absorber and to make corrections for the effects of absorption In this case, to obtain valid results the gain controls should be set at positions that prevent malfunction, or “overloading” of the system from the large echo signals Overloading in the input circuits can still occur, however, depending on the design Since this procedure may introduce errors in the case of large aperture, or array transducers, it is not recommended 2.2.3 Working distance The small vessel Doppler test object or string Doppler test object (see 3.1.1) is convenient for this test The tissue-equivalent absorber may be removed only for working distances less than cm The lateral position of the transducer assembly is adjusted with respect to the moving portion of the small vessel Doppler test object while observing the signal level of the Doppler output on the selected Doppler output connector The position which maximises the magnitude of the Doppler output is located This process is repeated over a range of separations between the transducer and the moving portion of the Doppler test object The effective spacing between the face of the transducer assembly (measured on the centre line axis of the assembly) and the intersection of the centre line axis of the transducer assembly and the moving portion of the Doppler test object is the working distance If the system includes an automatic gain control circuit, the Doppler output may be relatively constant over a large range of distances The working distance should be taken as the approximate centre of this flat region 2.2.4 Zero-signal noise level For future reference, the level of the noise components which are found at the Doppler output connector when the moving portion (string) of the Doppler test object is stopped should be measured using a true-r.m.s responding power meter, or visually on each output device The observer should be sure that stray reflections within the Doppler test object not influence this test (see 3.1.7) The passband of the power meter should extend over the full frequency range measured for the response of the particular Doppler output being tested (see 2.3.1) 2.3 Doppler frequency response Frequency response tests may be made by using a Doppler test object appropriate for the intended clinical use of the system positioned at the standard working distance Response and accuracy are preferably tested with the small vessel or string Doppler test object since these produce a single Doppler frequency which is readily measured, even visually on spectrum analyzers System control settings or ranges intended for arterial occlusive diagnosis should be used for tests with this Doppler test object System configurations designed for venous diagnosis may be tested using the large vessel or band Doppler test object The disk Doppler test object should be reserved for the distortion test specified in 2.3.3.1 2.3.1 Frequency response range The speed of the moving member (or fluid) in the Doppler test object is changed to produce a range of Doppler frequencies The time-average Doppler output is measured as a function of Doppler frequency or speed of movement, using an r.m.s or average responding voltmeter and a frequency counter, or other speed-indicating device If the Doppler output has one maximum value, the low-frequency response frequency and the high-frequency response frequency are found from those frequencies at which the output voltage is 0,707 times its maximum value, although other limits may be used if so declared This same procedure should apply in the case of multiple-peaked response curves where the minimum values between the maxima are not less than 0,707 times the voltage at the greatest maximum If the response curve is multiple-peaked (as it generally will be when using loudspeaker output tests) then the smallest value found between the peaks should be taken as defining the minimum detectable signal level A horizontal line on the graph at this signal level will then intersect the frequency response curve at this minimum and two other points These two other points are the low- and high-frequency response values and should be quoted as the result of the test, qualified by a statement of the level of this minimum relative to the highest value © BSI 01-2000 EN 61206:1995 The Doppler frequency output indication of the directional sensing systems should be zero The actual value, expressed as a percentage down from the output obtained when only one of the moving members is stopped, is the unbalance The maximum value found for the speeds of the moving member of the Doppler test object that produce Doppler frequencies within the range found using the procedure given in 2.3.1 should be recorded A Doppler frequency output for direction resolving systems is observed first with only the appropriate member moving, and then with both members moving at the same speed The change in indicated Doppler frequency should be reported as a percentage of the indication with one member moving The maximum percentage value found for moving member speeds that produce frequencies within the range found in 2.3.1 should be recorded 2.7 Response to Doppler spectrum Derived outputs which obtain information from the Doppler spectrum resulting from different velocities of blood flow within a given blood-vessel are to be tested using the flow Doppler test object (or volume-flow generator) described in 3.1.6 This Doppler test object provides a flow stream inside a tube which is to be mounted as is the string or band in the Doppler test objects described in 3.1.1 and 3.1.2 The tests are to be made at the working distance 2.7.1 Volume-flow circuits Systems intended for relative and absolute volume-flow measurements should be tested by using as a standard the volume-flow determined by “stopwatch and bucket” collection or from a flowmeter so calibrated The test will use the flow Doppler test object described in 3.1.6 The range of blood-vessel inner diameters for which the system is designed should be stated and tests made with test sections of tubing in the water tank which cover this same range The tests should cover the range of angles between the system sensitivity axis and the centre line of the vessel from 30° to 60°, and a range of Doppler frequencies covering the range found in the tests specified in 2.3.1 Results may be reported as the maximum deviation between the measured output and a straight line fitted to the data by the least squares method 1) The 10 2.7.2 Maximum-frequency followers Circuits which derive the maximum frequency of the Doppler spectrum should be tested using the flow Doppler test object and a liquid with viscosity equal to that of blood The maximum Doppler frequency indication produced by the system under test is to be compared with the maximum Doppler frequency which would be generated theoretically from a parabolic flow profile In parabolic flow, the peak-flow velocity is equal to twice the average flow velocity observed in the Doppler test object Average-flow velocity is obtained by dividing the volume-flow rate by the area of the test tubing Theoretical maximum Doppler frequency is derived from the formula: maximum Doppler frequency = (4I/2) cos where is the wavelength of the ultrasound in the fluid material within the tubing; is the angle between the sound beam and the tubing section; I is the average speed of the fluid Section Special doppler test objects 3.1 Doppler test objects The special Doppler test objects described in 3.1.1 to 3.1.6 are specified in terms of some of their performance characteristics at present, with tentative constructions suggested It is expected that future standards will specify the construction of these devices in more detail 3.1.1 String Doppler test object The string Doppler test object, shown in Figure 1, has a moving cylindrical member whose small surface roughness acts as the source of moving “scatterers” Such a Doppler target generates a single Doppler frequency rather than the spectral characteristic of a flowing liquid or vibrating ball, and also is a small and practical target for reproducibly simulating very small blood-vessels See [1]1) This type of Doppler test object may consist of a string passing over three or four pulleys driven by a motor, preferably reversing, with an attached tachometer String velocity is calculated from the known motor speed and pulley diameter, or equivalent means The string is mounted in the sound beam according to the arrangement shown in the lower half of Figure figures in square brackets refer to the bibliography in Annex C © BSI 01-2000 EN 61206:1995 The sketch at the bottom of Figure is of the plane defined by the active part of the moving string and by the axis of the sound beam The transducer is moved along the diagonal member of a block of tissue equivalent Doppler test object material This material should have an attenuation coefficient equivalent to the average attenuation coefficient for human soft tissue, 0,5 to dB cm–1 MHz–1 The attenuation should be checked at intervals recommended by the manufacturer using the following procedure: Set up a test tank filled with water such that the block of material to be tested can be inserted between ultrasonic transmitting and receiving transducers acoustically coupled through the water The receiving transducer may be a hydrophone The output of the receiving transducer is connected to a signal measurement system such as an oscilloscope The transmitting transducer is driven by repetitive tone bursts at the frequency of interest Add the block under test and note the change in level of the electrical signal output of the receiving transducer This change (in dB) is the attenuation of the block Linear operation of the measurement system is assumed This may be verified by inserting an additional identical block and checking that the change in output is within 0,3 dB of that above The insertion loss or attenuation, Ba in dB, of the block of tissue equivalent material is determined from the output signal level change as given by: where Vout(0) is the output signal level without the block; Vout(1) is the output signal level with the block The sound beam, after passing through the material and striking the string, should be strongly absorbed to prevent echoes from the walls Provision should also be made for removing the tissue equivalent material and for substituting a strong reflector for performing the fixed-target rejection test specified in 2.3.3.2 © BSI 01-2000 The space or distance between the string and the tissue equivalent material may be enlarged by use of a second block of tissue equivalent material as shown in the lower half of Figure The second material, if it occupies half the space between the wedge and the string, should have an attenuation coefficient equal to twice that of the first material A spacing of cm with a 0,5 cm thick block of the second material is suggested In this case, the range can be calculated from the equation given in Figure 1, where the quantities are defined in the figure A value of angle equal to 30° or less, is recommended as giving an adequate Doppler-shift frequency without selectively attenuating one edge of the sound beam relative to the other, and also allowing adequate space for the fixed-target reflector A problem with string Doppler test objects is vibration of the string, probably in the plane of the pulleys Vibration can introduce lower harmonics and spectral spreading, thereby degrading its quality as a single frequency Doppler test object This problem might be cured by providing more than one guide pulley, isolating motor vibrations, increasing the viscosity of the tank liquid, or changing the free-running length of the string The material for the string is still open for investigation Suggested materials include surgical silk, packing cord, monofilament nylon or other fishing line, silastic tubing, small-diameter rubber drive belts for portable tape recorders, or large “O” rings A principal problem is obtaining a string without a knot that produces a large transient signal A very long string can be used with data taken while the knot is out of the sound beam To use a string Doppler test object to simulate small blood vessels, the scattering strength should be chosen to be the same as a small blood vessel The size of blood vessel chosen should be stated on the label of the Doppler test object 3.1.2 Band Doppler test object This Doppler test object is identical to the moving string Doppler test object except that a band of finite width is used instead of a string It is designed to produce a single Doppler-shift frequency, but from a scattering surface which is as wide as the widest commonly encountered arteries or veins A width of 1,5 cm is suggested The general arrangement of the band Doppler test object in a three pulley drive situation is the same as the string Doppler test object as shown in Figure The requirements for the transducer mounting and adjustments are the same as given for the string Doppler test object The amplitude of band vibrations, however, should be very much less than that for the string 11 EN 61206:1995 3.1.3 Disk Doppler test object A Doppler test object consisting of an appropriate blood-equivalent scattering material is shown in Figure The purpose of the disk Doppler test object is to simulate a vessel which is wider than the transmitter sound beam and thus to produce the maximum strength of Doppler-shifted backscattering The material for the disk should have the same reflection strength in the MHz to 10 MHz region as does a cm thick slab of whole human blood The transducer could be positioned relative to this Doppler test object as shown in Figure 2, with the same mounting considerations as outlined in 3.1.1 for the string Doppler test object The entire incident sound beam should intersect the disk and not extend beyond the disk edge To maintain a narrow spectrum, the total width of the sound beam should be less than about 10 % of the radial distance measured between the axis of rotation of the disk and the centre of the sound beam 3.1.4 Piston Doppler test object The piston Doppler test object is designed to duplicate the back-and-forth motions of surfaces such as those of blood-vessels or of the pulsating heart, and is shown schematically in Figure [2] The reflection strength and range of motion of the piston material should be chosen to approximate that of the structure of interest The displacement of the piston of the Doppler test object is calculated from the dimensions of the driving system or by direct measurement of displacement The pulsations can be at the rate of s–1 to s–1 and need not be accurately sinusoidal 3.1.5 Small ball test object Another type of Doppler test object which utilises an oscillating target is the small ball Doppler test object This consists of a strongly reflecting small ball with a diameter of typically mm which is made to vibrate with small amplitude (1 4m) by a loudspeaker The echo-signals returned by the vibrating sphere will be modulated in phase with respect to the reference signal of the Doppler ultrasound system This phase modulation will be detected as a Doppler frequency which equals the frequency of the loudspeaker The detected Doppler output will be at a maximum when the signal received from the sphere is 90° out of phase with respect to the internal reference signal of the Doppler ultrasound system By moving the sphere in such a way that the condition of 90° out of phase is met a number of times, the sensitive volume of the Doppler ultrasound system can be deduced from the envelope of these maxima (see [3]) 12 As a consequence of the complex nature of the scatter from small sphere targets, and in particular its variation with frequency, the small ball Doppler test object is not recommended for use in pulsed Doppler ultrasound systems or systems with narrow (comparable to or less than mm) beam widths until its performance limits have been evaluated 3.1.6 Flow Doppler test object The flow Doppler test object is designed to produce a spectrum of Doppler frequencies as produced by blood in a natural blood-vessel Since the flow profile in vessels within the body is not parabolic and varies throughout the cardiac cycle, it is very difficult to simulate in a Doppler test object The usual compromise is to aim to achieve a parabolic profile since it is reproducible The part of the flow Doppler test object which is used for the tests should be mounted in the water tank in the same relative position with respect to the transducer as for the other Doppler test objects The Doppler test object, shown in Figure 3, includes a pump, reservoir and settling tank providing a gravity head for the flow system Particulate matter and air bubbles are removed by a filter, if necessary, and flow is conducted through a straight, non-expanding flow section through the test tubing in the water bath This section should be long enough to establish a parabolic flow profile with the recommended fluid The fluid may be collected in a sump for recirculation by a pump The outlet should be provided with a switchable stopcock leading to a graduated vessel Volume-flow calibration is accomplished by collecting and measuring the volume of fluid passed by the system over a timed interval If practicable, an electromagnetic flow probe or other flowmeter may be attached to the system to provide dynamic flow indications for later addition of pulsatile flow generators, or for more convenient use The pump and tubing should be carefully chosen to avoid cavitation of the liquid Such bubble generation can occur with too high a pump speed or from the presence of any tubing section which has an increasing cross-sectional area in the direction of flow Flow disturbances from tubing connectors should also be minimised © BSI 01-2000 EN 61206:1995 3.1.6.1 Fluid 3.1.7 Water tank (or gel block) The blood-simulating fluid should consist of water or a material of approximately the acoustic impedance of blood containing particulate scatterers, the whole having the scattering strength of whole human blood Suggested scatterers are polystyrene beads, paraffin (mineral) oil emulsion, Sephadex beads in water, or starch particles Glycerine is added to reach the viscosity of blood The fluid may be degassed as specified in 3.1.7 Tests using the Doppler test objects described in 3.1.1 to 3.1.6 should be conducted with the transducer surfaces and Doppler test objects in a water tank maintained at the temperature specified for the Doppler test object This section will also apply to Doppler test objects in which the water is replaced by a block of tissue equivalent gel, with holes in place of tubing The tank should be lined with sufficient acoustic absorbing material so that the tests are independent of position of the transducer and Doppler test object in the central region of the tank A simple test for the presence of stray tank wall or surface reflections, or bubbles, is to move the sound beam the minimum amount necessary to just eliminate the Doppler output caused by the Doppler test object, while observing them on a spectrum analyzer output device The motor drive, pumps or vibrator are to be kept running The remainder of the Doppler output will be caused by motion induced by these driving devices, and will exceed the noise level determined in accordance with 2.2.4 if reflections exist An additional test is to move the water surface or tank liner by one half-wavelength or more and observe the total output indication to see if it experiences a significant change This test can be applied to test Doppler test objects embedded in a block of gel if the surfaces are exposed Solid block Doppler test objects are best explored for internal reflections with a pulse-echo diagnostic system These reflections must be weaker than the reflection from the simulated blood-vessel for testing continuous-wave flowmeters Water can be degassed by raising its temperature to boiling, followed by cooling to room temperature, or by applying a vacuum while shaking the fluid Subsequent transfer to the test tank should be made without entraining or trapping air bubbles The procedure should be repeated weekly or when the system noise level increases 3.1.6.2 Test tubing The tubing should have a known and uniform inside diameter The walls should introduce minimal beam attenuation, and beam distortion as a result of attenuation, refraction and critical angle reflections These effects lead to a loss of low-frequency components in the Doppler spectrum A suggested wall material is dialysis (cellophane) or similar tubing in sufficiently small diameters A sound speed in the wall less than that in the fluid, or a construction with the fluid passing through a hole in a block of tissue equivalent material minimises or avoids refraction of the sound beam which otherwise causes loss of low frequencies in the Doppler spectrum Any wall material should be tested by observing the Doppler output on a spectrum analyzer The level should be constant at frequencies above the lower cut-off frequency in the Doppler ultrasound system used when the fluid can be guaranteed to have laminar flow and the sound beam fills the tubing © BSI 01-2000 13 EN 61206:1995 Figure — Schematic diagram of a string Doppler test object 14 © BSI 01-2000 EN 61206:1995 Figure — Schematic diagram of band, disc and piston Doppler test objects © BSI 01-2000 15 EN 61206:1995 16 Figure — Schematic diagram of a flow Doppler test object with pump return © BSI 01-2000